I've used the POR 15 on most of my inner panels over a sand blasted finish and it sticks great. I read on the can that if you want to paint over the por 15 you should spray a thin coat of primer over the por15 when it gets tacky. I would recomend painting it directly on the metal. One of the great things about the por15 is that it can bend with the metal. I would think the primer under the por would chip if it flexed or if you hit it by axident.
